A dump truck is a type of carrier vehicle that is normally equipped with a body disposed on the truck frame. The body, which can hold a heavy load, such as crushed rock, can be tilted in order to dump its contents. A traveling drive device that drives the drive wheels of the dump truck includes a tubular motor housing sleeve attached to the chassis and a drive motor such as an electric motor or a hydraulic motor that is disposed inside the motor housing sleeve and rotationally drives a rotating shaft. In addition, a tubular spindle is disposed toward the front end of the motor housing sleeve with a wheel mounting sleeve rotatably mounted via a bearing at the outer circumference of the tubular spindle. Wheels are fixed onto the exterior of the wheel mounting sleeves. A traveling speed reduction gear unit that slows down the rotation of the drive motor and transmits the slowed rotational force to the wheel mounting sleeve (wheels) and the like are disposed inside the wheel mounting sleeve (see, for instance, Japanese Laid Open Patent Publication No. S62-221918 and Japanese Laid Open Patent Publication No. 2006-264394).
Lubricating oil used to keep the various gears and the like constituting the reduction gear mechanism in a lubricated state is collected inside the wheel mounting sleeve. As the vehicle travels, the temperature of the lubricating oil rises and the viscosity of the lubricating oil is reduced, resulting in a lower level of lubrication performance. In order to sustain the required level of lubrication performance and prevent degradation of the lubricating oil, an oil cooler is installed in the circulation path. The lubricating oil is circulated via a lubricating oil pump disposed outside the wheel mounting sleeve so that the lubricating oil, forced to circulate via the circulation path into and out of the wheel mounting sleeve, is cooled at the oil cooler.
An intake piping is connected to the intake side of the lubricating oil pump and the intake piping is inserted through the wheel mounting sleeve. An intake port of the intake piping, positioned so as to face toward the bottom surface of the wheel mounting sleeve, extends to a position at which it is immersed under the surface of the lubricating oil in the wheel mounting sleeve. A supply piping is connected to the outlet side of the lubricating oil pump. This supply piping, too, is inserted through the wheel mounting sleeve so as to supply the lubricating oil from the lubricating oil pump into the wheel mounting sleeve.